This disclosure relates to copolymer formulations, methods of manufacture thereof and to articles comprising the same. In particular, the present disclosure relates to self assembled nanostructures obtained by disposing a block copolymer over a polymeric layer that exists in the form of a brush or a mat.
Block copolymers form self-assembled nanostructures in order to reduce the free energy of the system. Nanostructures are those having average largest widths or thicknesses of less than 100 nanometers. This self-assembly produces periodic structures as a result of the reduction in free energy. The periodic structures can be in the form of domains, lamellae or cylinders. Because of these structures, thin films of block copolymers provide spatial chemical contrast at the nanometer-scale and, therefore, they have been used as an alternative low-cost nano-patterning material for generating periodic nanoscale structures. This method of achieving periodic structures is termed directed self-assembly (DSA). In directed self-assembly, small pitch sizes are achieved by using the self-assembled block copolymer to lithographically pattern a substrate.
Copolymers that are primarily used in directed self-assembly are polystyrene-polymethylmethacrylate block copolymers and random polystyrene-polymethylmethacrylate copolymers that are used as a brush to control the surface energy of a substrate upon which the block copolymer is disposed. The directed self-assembled method used often involves disposing a cross-linked polystyrene mat 104 on a substrate 102 as shown in the FIG. 1. Following the crosslinking of the polystyrene mat 104, conventional lithography using a photoresist is used to make an array of lines 106 on the polystyrene mat 104. The lines 106 are then subjected to an etch process which trims them to a smaller dimension and removes the polystyrene mat 104 between the array of lines 106. The space between the lines 106 is then backfilled with the random polystyrene-polymethylmethacrylate copolymers 108 (hereinafter termed the backfill 108) to produce a brush coating. Following this, the polystyrene-polymethylmethacrylate block copolymer 110 is disposed on the backfill 108 and the strips of the polystyrene mat 104. The polystyrene mat strips 104 and the brush backfill 108 help anchor the domains of the polystyrene-polymethylmethacrylate block copolymer 110, as shown in the FIG. 1. From the FIG. 1, it may be seen that the polystyrene mat strips 104 are vertically aligned with the polystyrene domains of the polystyrene-polymethylmethacrylate block copolymer 110.
During directed self assembly, it is desirable to have the size of the polystyrene mat strips 104 match the size of the polystyrene domains in polystyrene-polymethylmethacrylate block copolymer 110 as shown in the FIG. 1 or 2. The FIG. 2 is an expanded version of the FIG. 1 showing the steps of lithography, etching and trimming, followed by the brush backfill and the consequent directed self assembly. The FIG. 2 reflects the end result of a desirable directed self assembly process. In particular, it may be seen that the etching that occurs after the directed self assembly produces equally spaced channels 112 in the substrate 102. However, this is not what occurs when crosslinked polystyrene is used as a mat as is detailed below in the FIG. 3.
In particular, as seen in the FIG. 3, the combination set (i.e., the polystyrene mat and the photoresist) suffers from a limitation in that the crosslinked polystyrene mat is much more etch resistant than the photoresist. This prevents the polystyrene mat strips 104 from being the same size as the polystyrene (lamellar or cylindrical) domains of the polystyrene-polymethylmethacrylate block copolymer 110. The differential etching rates between the crosslinked polystyrene and the photoresist produces non-uniform domains (i.e., trapezoidal shaped crosslinked polystyrene domains) in the crosslinked polystyrene mat. As a result of this non-uniform domain production, the resulting etched channels in the substrate have a bimodal distribution, which is undesirable.
It is therefore desirable to have a system (i.e., a mat composition and a brush composition) that facilitate the formation of uniform channels in the substrate during directed self assembly.